The present invention relates generally to a system and method of operation for controlling a wind turbine, and in particular a system and method of controlling a wind turbine located in geographic regions having permafrost.
In colder northern climates and certain areas of higher elevation, there are regions where the subsurface soil remains frozen year-round. This frozen soil phenomena is commonly referred to as permafrost. Permafrost formed over hundreds of thousands of years in regions where the average yearly air temperature is less than 0° C. In arctic and sub-arctic regions, such as the Alaskan Arctic Plain the permafrost may be up to 650 meters thick and have a soil temperature between −9 to −11° C.
While the soil remains frozen in permafrost regions, a portion of the soil, referred to as the active layer, will thaw during summer months and then refreeze during the winter. The depth of the active layer will vary depending on the latitude of the location, the local geography and the weather. During the summer, the active layer will typically be between 0.5 to 4 meters thick. Due to high levels of ice content in the active layer, depending on the drainage conditions of the area, the active layer may become over saturated with water. Where saturation occurs, trees and other vegetation may die or tilt due to a lack of support by the soil.
Structures that are built in permafrost regions need to account for the seasonal variability in the soil condition. Commonly, structures are built on pilings that are driven past the bottom of the active layer into area of the soil that remains frozen year round. In this way, even if the active layer becomes over saturated with water during the summer, the structures and buildings remain stable.
Due to recent global climate changes, which are expected to continue, annual average temperatures have risen steadily. As a result of these climate changes, the regions where permafrost is found will continue to recede into colder areas. Even in areas where permafrost remains, the active layer will grow in depth with the increased temperatures. The increase in size of the active layer may cause issues for structures located in permafrost regions, such as buildings, roads, electric power transmission lines and other tall structures such as wind turbines. Further, structures may have foundational issues as the increase in the active layer reduces the length of the pilings that are supported by permafrost. These structural issues are magnified in tall structures such as wind turbines the change in foundational support may also change the operational characteristics of the wind turbine.
While existing wind turbine control systems are suitable for operating in arctic regions having permafrost, there remains a need for improvements, particularly regarding the control of wind turbines to account for changing soil conditions.